The present invention relates in general to automotive electrical wiring systems, and, more specifically, to a method and computer-aided tool for designing a wiring harness in a manner that optimizes performance in handling short circuits while minimizing the size and weight of the wires.
Because of the large number of electrical devices and accessories in a typical automotive vehicle, the electrical wiring is a significant element of the cost, weight, and manufacturing considerations of the vehicle. Various sections of the wiring are often grouped into a wiring harness in which a plastic or tape covering is applied to protect and organize sections of the wiring harness and to facilitate routing of the wires through the vehicle. Each wire in the wiring harness has a diameter (known as its gauge size) which is selected to be large enough to carry the current expected to flow in its respective circuit. It is desirable to use the smallest wire size (i.e., largest gauge number) for each respective wire segment due to the relatively high cost of copper and the need to reduce vehicle weight for maximizing fuel economy.
A typical vehicle requirement that must be met by a wiring designer relates to the ability of the wires to carry both a short circuit current and a steady state current without exceeding the insulation performance rating. In order to ensure that the wiring does not fail during a worst case short circuit, any potential design must be evaluated with respect to fuse and wire performance during a short circuit. One such typical requirement relates to the amount of time required for a fuse to blow during a short circuit. The longer the time that it takes for the fuse to blow, the greater the heat produced and the greater the temperature that the wiring must withstand during the short circuit.
Common fuses used in automotive electrical circuits include J-case fuses and mini-fuses. A typical manufacturer's requirement is that for any circuit in which a blow time for a J-case fuse exceeds 1.0 seconds or a blow time for a mini-fuse exceeds 0.5 seconds, then the wiring must be designed to withstand 135% of the rated current of the fuse (as determined at the maximum temperature rating of the insulation). For example, a 10-amp fuse to be located in the vehicle exterior (e.g. engine compartment) would be required to withstand 13.5 amps at 105° C. On the other hand, if the fuse blow time is less than the specified times, no increase in wiring size is required by this rule.
In conventional vehicle electrical system development, the circuit wiring is often sized before all other circuit details are known. A wiring designer typically uses a best guess in order to select a wiring size that will likely be demonstrated to meet all required specifications once all circuit details are finalized and a full computer-aided modeling analysis is performed on the electrical system to ensure proper short circuit performance, steady state current carrying capability, and satisfaction of the fuse blow time requirement. Consequently, over-sized wire gauges are often adopted by the wiring designer at the beginning of a vehicle design in order to protect for unknown factors that arise later during the development.